Data marshalling support

These routines allow C code to work with serialized objects using the same data format as the marshal module. There are functions to write data into the serialization format, and additional functions that can be used to read the data back. Files used to store marshalled data must be opened in binary mode.

Numeric values are stored with the least significant byte first.

The module supports several versions of the data format; see the Python module documentation for details.

The following exceptions can be raised by these functions: ValueError if the value cannot be marshalled, ValueError or TypeError if the data is malformed, EOFError if the end of the data is reached before the value is complete, OSError if reading from or writing to a FILE* fails, KeyboardInterrupt if reading or writing is interrupted by a signal, and MemoryError if memory allocation fails.

Changed in version 3.16.0a0 (unreleased): Previously, in functions taking a FILE*, the reading functions raised EOFError instead of OSError and KeyboardInterrupt, and the writing functions ignored I/O errors and interruptions.

Py_MARSHAL_VERSION

The current format version. See marshal.version.

void PyMarshal_WriteLongToFile(long value, FILE *file, int version)

Marshal a long integer, value, to file. This will only write the least-significant 32 bits of value; regardless of the size of the native long type. version indicates the file format.

This function can fail, in which case it sets the error indicator. Use PyErr_Occurred() to check for that.

Note

In Python 3.15 and earlier, this function does not set the error indicator if writing to file fails. The same caveat applies as for PyMarshal_WriteObjectToFile().

void PyMarshal_WriteObjectToFile(PyObject *value, FILE *file, int version)

Marshal a Python object, value, to file. version indicates the file format.

The marshalled representation of value is built in memory and then written to file. Nothing is written if value cannot be marshalled.

This function can fail, in which case it sets the error indicator. Use PyErr_Occurred() to check for that.

Note

In Python 3.15 and earlier, this function can write only part of the data to file and return without setting the error indicator, for example if value contains an object which cannot be marshalled or if writing to file fails (see gh-155907). Code which has to support these versions cannot rely on PyErr_Occurred() alone to know that all the data was written. It can use PyMarshal_WriteObjectToString() and write the result itself, or validate what was written.

Changed in version 3.16.0a0 (unreleased): Previously, the data was written to file as it was produced, using a buffer of constant size, and incomplete data was written if value could not be marshalled.

PyObject *PyMarshal_WriteObjectToString(PyObject *value, int version)

Return value: New reference.

Return a bytes object containing the marshalled representation of value. version indicates the file format.

On error, raises an exception and returns NULL.

The following functions allow marshalled values to be read back in.

long PyMarshal_ReadLongFromFile(FILE *file)

Return a C long from the data stream in a FILE* opened for reading. Only a 32-bit value can be read in using this function, regardless of the native size of long.

On error, raises an exception and returns -1.

int PyMarshal_ReadShortFromFile(FILE *file)

Return a C short from the data stream in a FILE* opened for reading. Only a 16-bit value can be read in using this function, regardless of the native size of short.

On error, raises an exception and returns -1.

PyObject *PyMarshal_ReadObjectFromFile(FILE *file)

Return value: New reference.

Return a Python object from the data stream in a FILE* opened for reading.

On error, raises an exception and returns NULL.

PyObject *PyMarshal_ReadLastObjectFromFile(FILE *file)

Return value: New reference.

Return a Python object from the data stream in a FILE* opened for reading. Unlike PyMarshal_ReadObjectFromFile(), this function assumes that no further objects will be read from the file, allowing it to aggressively load file data into memory so that the de-serialization can operate from data in memory rather than reading a byte at a time from the file. Only use this variant if you are certain that you won’t be reading anything else from the file.

On error, raises an exception and returns NULL.

PyObject *PyMarshal_ReadObjectFromString(const char *data, Py_ssize_t len)

Return value: New reference.

Return a Python object from the data stream in a byte buffer containing len bytes pointed to by data.

On error, raises an exception and returns NULL.